Earliest-diverging rhynchocephalian from Gondwana: a new sphenodontian from the upper triassic los colorados formation (northwestern Argentina)

Abstract

Rhynchocephalia is one of the two sister clades that comprise Lepidosauria. The oldest known record is from the Middle Triassic (Anisian) of Laurasia whereas their earliest Gondwanan record is from the Late Triassic. The few known records come from the Brazilian Candelaria Sequence and the Marayes-El Carrizal Basin in Argentina. Here we describe a new sphenodontian from the Los Colorados Formation, the first one for the Ischigualasto-Villa Unión Basin (northwestern Argentina): Coloradosphenodon lahni gen. et sp. nov. known from a single partial skull including most of the lower jaws. Our phylogenetic analyses consistently depict Coloradosphenodon lahni as a non-eusphenodontian acrosphenodontian, representing the earliest-diverging rhynchocephalian from Gondwana. It is closely related to Parvosaurus harudensis from the Late Triassic (Norian) Arnstadt Formation (Germany). We present the first quantitative biogeographical analyses for Rhynchocephalia, which suggest that European basins played a key role in the origin and diversification of the clade. Disparity analyses results indicate that rhynchocephalians attained their maximum cranial morphological disparity during the Late Triassic, with early rhynchocephalians being the more morphologically disparate taxa. Our findings add to the scarce Triassic record of rhynchocephalians in Gondwana and provide new insights into their early diversity and evolutionary history.